1. Field of the Invention
The present invention relates to a clock generator used by electronic devices and microprocessors as the operational clock, more specifically to a clock generator that reduces undesired radiation noises (electro magnetic interferences: EMI) caused by the operational clock of a high frequency by modulating the cycle (spreading the spectrum) of the clock.
2. Description of the Related Art
Nowadays, a great many electronic devices use microprocessors and digital circuits, which have clock generators inside the circuits, and the electronic devices operate the circuits based on the clocks generated. In recent years, the frequency required for the operational clock is raised accompanied with the speeding up of operation, and the undesired radiation noises (EMI) emitting from high-speed microprocessors have been recognized as a serious problem. Some administrative agencies have moved to regulate such EMI noises. For example, the FCC (Federal Communications Commission) of US provides the test procedure for measuring the EMI noises emitting from apparatuses and the maximum permissible emission of the EMI noises, as part of the regulations to the apparatuses that generate high frequencies. In order to observe such regulations to the EMI noises by the administrative agencies, EMI countermeasures have been made in various manners. For example, development of high frequency suppressing elements, shielding of an apparatus that generates the EMI noises (metal shielding of the surroundings), and examination of circuits capable of suppressing emissions of the EMI and so forth have been pursued. However, these methods require many technical difficulties for suppressing the EMI emissions in any of them, and tremendous efforts and costs to overcome these problems, including that a large circuit board is required especially in the case of using the shielding method.
Now, as another approach to reduce the EMI noises, the method of varying a clock signal itself has been examined. As the method of varying the clock signal itself, the method of modulating the clock frequency in a manner to spread the spectrum is known, and the following patent document 1 can be cited as an example.
[Patent Document 1]
Japanese Published Unexamined Patent Application No. Hei 9-98152
However, the method disclosed in the Patent Document 1 (spread spectrum clock generator) premises controlling the oscillation frequency of a VCO (Voltage Controlled Oscillator) by a control voltage generated in accordance with a frequency modulation profile, and uses frequency dividers to a reference clock in the process of generating the control voltage. Accordingly, this method lowers a phase comparison frequency, and is likely to be influenced by disturbances of noises and so forth. Therefore, it was difficult to attain such a profile that the phases of the reference clock and a spectrum spread clock based on the reference clock have a desired periodic characteristic precisely.
And, in the case of configuring one system composed of plural devices and circuit systems, considering a case that applies a spectrum spread clock to the operations of the parts of devices and circuits inside the system, it will involve plural clocks having the phases not ensured to each other inside one system. To maintain the operation appropriately in the system using the operational clocks as shown in the patent document 1, it is necessary to design a data transfer interface between devices to be able to operate in non-synchronization. This requires an additional buffer memory.
FIG. 10 is a block diagram of a conventional data processing circuit using a spectrum spread clock, which incorporates a buffer memory.
As shown in FIG. 10, a circuit block 20 accepts input data through a flip-flop 24 based on a reference clock Rclk generated by a reference clock generator 22, and stores the data in a RAM 26.
The data stored in the RAM 26 are read based on a spectrum spread modulation clock Mclk generated by a spectrum spread modulation clock generator 23, and are supplied to a flip-flop 25. The flip-flop 25 transfers the data to a circuit block 21 on the side of a user based on the spectrum spread modulation clock Mclk.
In this manner, the conventional operation of the devices based on the spectrum spread modulation clock has needed a buffer memory (RAM 26 in FIG. 10) that temporarily holds the data, in order to avoid malfunctions resulting from that individual operational clocks come into a non-synchronous state. Accordingly, the cost is increased, and the circuit space for the buffer memory becomes necessary. Further, the constraints such as a clock speed for driving the buffer memory and so forth make the circuit design more difficult.